The present invention relates to a switching frequency generator for supplying a switching frequency, and more particularly to a switching frequency generator suitable for use in a PWM (Pulse Width Modulation) type switching device, and a magnetic resonance imaging (MRI) system using the same, which is, for example, a PWM type gradient power source or clock circuit of an MRI system.
In magnetic resonance imaging, an object to be diagnosed is placed in a static magnetic field, whereby atomic nuclei align themselves with the static magnetic field. Then gradient magnetic fields in three x-, y- and z-directions are applied to the object for spatially encoding and a radio frequency (RF) signal is applied to the object for exciting the atomic nuclei in a magnetically sliced plane, which has a certain thickness in a slicing direction, of the object. When the RF signal is removed, magnetic resonance (MR) signals emitted from the sliced plane can be collected. A series of the excitation and MR signal acquisition is performed on a predetermined pulse sequence. The collected MR signals are then processed, for example, by Fourier transformation to form image data of the magnetically sliced plane of the object.
An MRI system, which is generally used and carries out the above procedure, has a gradient power source to supply power to gradient coils. Thus gradient magnetic fields are applied to an object through the coils.
One type of the gradient power source uses a pulse width control system. Such gradient power source has, for example, an operation circuit for computing the difference between a reference voltage signal and a detected voltage signal detected as the representative of a gradient magnetic field current. A PWM circuit varies a duty ratio by using the output of the operation circuit, a switching circuit which is turned on and off by the output of the PWM circuit, and a smoothing circuit connected between the switching circuit and the gradient coils. Among these circuits, the PWM circuit includes an external oscillator or an internal oscillator and conducts a pulse width control by using the oscillation output as a synchronizing signal. In this case, the oscillation frequency of the external or internal oscillator is fixed.
In general, a circuit which performs switching operation, e.g., the above-mentioned PWM circuit, produces an output which inherently contain higher harmonics of frequencies which is multiples of the basic frequency. Such higher harmonics often causes noise. This is attributable to the fact that an MRI system, in a resonance frequency band of a measuring object such as hydrogen in the magnetic field, is equivalent to a receiver having very high sensitivity. For instance, representing the resonance frequency of an MRI system by f.sub.0, while representing the switching frequency by f', a higher harmonic or harmonics (5f' in the illustrated case) may fall within the image frequency band whose center frequency is the resonance frequency f.sub.0 as shown in FIG. 1. In such a case, a linear artifact called "F1 noise" is generated in the MRI reconstructed image.
The gradient power source of the pulse width control type, of the prior art however, cannot avoid generation of F1 noise, due to the fact that the frequency of the synchronizing signal supplied to the PWM circuit is fixed as stated before.
The F1 noise attributable to the spurious characteristic would be avoided if the switching system is not used. Such a solution, however, requires a large-sized expensive series regulator or shunt regulator, and the gradient power source to be of linear type which is low in efficiency.
In the known MRI system, the F1 noise attributable to spurious characteristic is caused not only by a switching unit such as the gradient power source but also by clock units of digital circuits. In addition, since frequencies are independently determined for different units, a plurality of higher harmonics 5f', 3f.sub.a, 2f.sub.b, come into the image frequency band so as to form a plurality of linear F1 noises, as shown in FIG. 2.